The number of workers on zero-hours contracts has increased by a fifth over the past year, official figures show.

Just over 900,000 people say they work on a zero-hours basis in their main job - up from 747,000 a year ago.

The Office for National Statistics (ONS) said its figures showed that almost 3% of UK workers are on zero-hours contracts.

The controversial contracts, under which workers do not know how many hours they will work from week to week, has been under the spotlight this week after retail giant Sports Direct said it would change arrangements for some of its staff.

Nick Palmer, from the ONS, said: "The estimated number of people saying they work on a zero-hours contract has risen by over 20% since the same time last year.

"The ONS will continue to monitor and report on this trend to help inform understanding of changes in the UK's employment market.

"It is likely though that some of the increase we are seeing is because public awareness of the term 'zero-hours contract' has continued to grow."

Women make up 55% of those on zero-hours contracts, while one in five of those on the contracts is in full-time education.

TUC general secretary Frances O'Grady said: "Zero-hours contracts have become an easy way for bosses to employ staff on the cheap. There is no getting away from the fact that zero-hours workers earn less money and have fewer rights than people with permanent jobs.

"It is very easy for politicians and employers to talk about the 'flexibility' these contracts offer, but they are not the ones living at the sharp end of the labour market.

"If you don't know how much work you will have from one day to the next, paying the bills and arranging things like childcare can be a nightmare.

"Today's figures are a stark reminder of why we need to create more decent jobs people can actually live on."

New TUC analysis showed that the typical employee earns 50% more an hour than a typical worker on a zero-hours contract.

The median hourly rate for a zero-hours worker is £7.25, while for all employees it is £11.05, the study showed.

A Business Department spokesman said: "As the Prime Minister has made clear, we want to do more to build an economy that works for everyone and to help working people who are struggling to get by.

"Since May last year, the use of exclusivity clauses has been unlawful, meaning that individuals have more control over their lives and can work more hours with another employer if they wish.

"Fewer than 3% of the UK workforce classes itself as being on a zero-hours contract in their main job, with almost 70% of those on this type of contract happy with the number of hours they work."

"The Government has taken some important steps to make work pay, including introducing the National Living Wage. The next step is to look at how it can make people's incomes more secure.

"For nine in 10 people, having a steady income is important - and this makes job security as important as the level of people's pay. There are currently 4.5 million people in some form of insecure employment from zero-hours contracts to variable shift patterns or agency work.

"Some people do value these more flexible ways of working as it can help them balance other responsibilities, but it is a different story for those who rely on this type of work as their main income.

"Not knowing how much you can earn from one month to the next can make it hard to pay the bills and keep up with other financial commitments, which can result in debt problems further down the line."

Conor D'Arcy, policy analyst at the Resolution Foundation think-tank, said: "Despite the welcome jobs recovery, the number of people on zero-hours contracts continues to rise, suggesting that they will remain a controversial landmark of the employment landscape for some time.

"It is concerning that much of the growth is among older workers, some of whom might struggle to balance their family budget if their income varies as a result of having no guaranteed hours.

"The Prime Minister could deliver on her powerful maiden speech critique of insecurity in the workplace by protecting the workers who get stuck on zero-hours contracts long term when they would prefer a fixed-hours contract.

"Banning zero-hours contracts is not the answer, as it punishes workers who genuinely prefer the flexibility they offer, but the part they play in the wider issue of insecure working suggests that they need far closer, and more measured, scrutiny."

Liberal Democrat business spokeswoman Lorely Burt said: "The most concerning aspect of these figures is that over half of zero-hours workers are under 25. While zero-hours can provide some flexibility to workers, they cannot form the basis of the long-term careers many young people crave.

"We need to create a formal right to request a fixed contract, guaranteeing that zero-hours contracts are an option for employees, not a tool for unscrupulous employers."

Unison general secretary Dave Prentis said: "These unfair contracts are a scourge on the workers stuck on them. Unpredictable working hours mean uncertain family finances, and anxiety that there will be no money for rent or food at the end of the week.

"Most home care workers are on zero-hours and unscrupulous care companies regularly use them to put undue pressure on their employees. If staff complain about working conditions or turn down a shift, they're often threatened with having hours taken away."

Unite general secretary Len McCluskey called on the Government to ban zero hours contracts.

He added: "This sharp rise in the numbers of people reliant on zero hours work points to an economy which is increasingly being built on precarious work with more and more workers not knowing from one week to the next whether they will be able to pay the bills."